In recent years, demand for transparent plastic substrates has been growing as substrates of hard coat films and antireflective films which are to be bonded to plastic articles, surface displays for LCDs and soon, indication screens and touch panels of cellular phones and mobile game machines to thereby impart rubfastness, or to glass products and display surfaces made of glass (CRT, PDP, etc.) to thereby achieve antiscattering, antireflective and stainproof effects.
Among all, there arises a surprising increase in the demand for polyester resin-based films, in particular, biaxially oriented polyethylene terephthalate (PET) films as the substrate films of the functional films as described above because of having excellent mechanical properties, burning resistance, chemical resistance and so on.
In the case of forming functional plastic films with the use of these transparent plastic substrates, it has been a practice to form a functional layer (for example, a hard coat layer) made of an organic compound resin and having a thickness of about several μm to about 50 μm on a substrate either directly or via a highly pressure-sensitive adhesive layer in the form of a thin film. The refractive index (in the facial direction) of a PET film is about 1.65, whereas the refractive index of a hard coat layer made of an organic compound such as an acrylic resin is generally around 1.53 and ranges from 1.50 to 1.56, thereby showing a difference in refractive index of 0.10 or more. As a result, there arise problems such as (1) showing a high reflectance at the interface; (2) interference spots occurring due to the interference of reflection on the interface and reflection on the surface; and (3) interference spots occurring due to the interference of the reflection on the back surface of PET. These three problems (1), (2) and (3) would worsen the visibility of an article such as an image display unit or damage the high-grade impression thereof.
Under a three-wavelength fluorescent lamp having a high ratio of bright line spectrum components, in particular, interference spots are highlighted. Recently, three-wavelength fluorescent lamps have become common for family use and thus the problem of interference spots have become serious. Therefore, use of functional plastic films having PET films as the substrate is severely restricted, otherwise functional films suffering from the problem of interference spots might be put into the market as such. In the field of large-sized flat TV sets provided with PET films as the substrate, interference spots are actually observed in most of antireflective films loaded thereon.
The occurrence of interference spots is caused by extremely subtle unevenness in film thickness of a hardening layer (a hard coat layer, etc.) or a pressure-sensitive adhesive layer of about several μm to about 50 μm formed on a substrate. Methods for overcoming this problem of the uneven film thickness is fundamentally different from the case of, for example, a hard coat layer of a synthetic resin lens wherein use is generally made of a film-forming method with little coating unevenness such as the immersion method or the deposition method. Concerning plastic films and functional plastic films which are usually produced in the form of a roll film of 30 cm or more in width and 10 m or more in length, it is impossible to overcome the problem of the extremely subtle coating unevenness occurring at random by using the existing coating methods.
To prevent these interference spots, attempts have been made to bring the refractive index of a hard coat layer close to the refractive index of a PET film by applying an ionization radiation-hardening resin containing ultrafine particles of a high-refractive index metal oxide on a high-refractive index film such as a PET film and hardening the same to form a hard coat layer, or by applying a siloxane-based thermosetting resin containing ultrafine particles of a high-refractive index metal oxide on a and hardening the same to form a hard coat layer. Although these methods are effective in lessening interference spots, they would bring about other problems such as a decrease in the mechanical strength of the hard coat layer, an increase in haze and, furthermore, a tendency toward an insufficient refractive index of a high-refractive index layer in the case of forming two or more antireflective layers on the surface.
Under these circumstances, there has been reported that interference spots occur in a hard coat film using a substrate made of PET and that these interference spots can be hardly prevented by the existing techniques without inducing serious side effects such as a decrease in mechanical strength or an increase in haze (see, for example, Ikuhiro Kimura, Hansha-Boshi Maku no Tokusei to Saiteki Sekkei Maku Sakusei Gijutsu, Gijutsu Joho Kyokai, 2001, pp. 166–171).
Next, the situation will be further described. Although here have been proposed methods of improving the mechanical strength (see, for example, JP-A-7-151902), the problem of haze still remains unsolved. In addition, few of the proposed methods can result in satisfactory improvement in the mechanical strength.
Also, there have been reported some countermeasures for interference spots by introducing scattering (see, for example, JP-A-8-197670 and JP-A-10-282312). Although these methods are effective as countermeasures for interference spots, the refractive index still remains at a high level and there arises an additional problem of haze, which severely restricts the practical use thereof.
Moreover, it has been reported that the occurrence of interference fringes can be prevented by providing, between a hard coat layer and a PET film, a buffer layer (3 to 50 μm) having an intermediate refractive index between the hard coat layer and the PET film (see, for example, JP-A-2000-347003). As the results of our follow-up test, however, only insufficient effects could be achieved on interference spots under the above conditions.
The above-described problem of interference spots occurring in the case of using a high-refractive index substrate typified by a PET film and a usual hard coat layer having a refractive index of about 1.53 also arises in the combination of a low-refractive index substrate typified by triacetylcellulose (TAC) and a high-refractive index hard coat layer. For example, antireflective hard coat films having a high-refractive index hard coat layer and a low-refractive index antireflective layer laminated on a substrate are widely known and there have been also disclosed antireflective hard coat films wherein a high-refractive index hard coat layer is formed on a TAC film and a low-refractive index an antireflective layer made of silicon oxide is further overlaid thereon (see, for example, JP-A-7-287102). It has been reported that interference spots occur even in the latter combination (see, for example, JP-A-2001-318207 (paragraph 0004)) and no effective countermeasure therefor has been proposed so far.
Furthermore, similar interference spots occur in the case where a pressure-sensitive adhesive layer is overlaid on a substrate and the refractive index of the substrate largely differs from the refractive index of the pressure-sensitive adhesive layer. To solve this problem, it has been proposed to elevate the smoothness of a PET film to thereby lessen the interference spots occurring due to the PET film and the pressure-sensitive adhesive (see, for example, JP-A-2001-071439), though only insufficient effects can be achieved on the interference spots thereby.
Moreover, it is frequently observed at present that a substrate film per se is produced in the state of being provided with a primer layer having a refractive index largely different from that of the substrate and thus the substrate film per se suffers from the problem of interference spots as described above, thereby hindering the solution of the problems of interference spots and high refractive index.